1. Field of the Invention
The present invention relates to a vibrating device including a piezoelectric portion, such as a piezoelectric element, and more particularly, to a piezoelectric pump which conveys fluid by causing a vibrating plate which defines a wall surface of a pump chamber to vibrate using the piezoelectric portion.
2. Description of the Related Art
In general, a piezoelectric thin film is sensitive to tensile stress and easily breaks when a tensile stress is applied thereto. Accordingly, a technique to prevent the breakage of the piezoelectric thin film due to the tensile stress by applying a residual stress in a compressive direction is known (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-146640).
The piezoelectric thin film is formed on a substrate having a higher coefficient of linear expansion than that of the piezoelectric thin film in a heated environment. Then, when the piezoelectric thin film is cooled, the substrate contracts by a greater amount than the amount by which the piezoelectric thin film contracts. As a result, a residual stress in the compressive direction is applied to the piezoelectric thin film.
FIGS. 1A and 1B are sectional views of a piezoelectric thin film or a piezoelectric element in a known structure.
A structure 200 is obtained by thermally bonding a piezoelectric layer 201 to a substrate 202 and performing a cooling process after the thermal bonding process. FIG. 1A is a sectional view of the structure 200 in a static state after the cooling process. When the structure 200 is in this state, a residual stress in a tensile direction is applied to the substrate 202 and a residual stress in a compressive direction is applied to the piezoelectric layer 201. In this structure 200, since the residual stresses applied to the piezoelectric layer and the substrate are different from each other, the piezoelectric layer expands and the substrate contracts in the structure 200. As a result, the structure 200 slightly warps. Due to the expansion of the piezoelectric layer, there is a large residual stress gradient in the piezoelectric layer 201. The residual stress in the compressive direction is extremely small in an area near the surface of the piezoelectric layer 201.
The above-described piezoelectric element or the piezoelectric thin film may be used in a vibrating device or a piezoelectric pump which vibrates the piezoelectric element or the piezoelectric thin film by applying an alternating voltage thereto. In such a case, when the piezoelectric layer vibrates, the piezoelectric layer receives a tensile stress or a compressive stress.
FIG. 1B is a sectional view of the structure 200 in the state in which the structure 200 is bent to be convex on the piezoelectric-layer side due to the vibration. In the structure 200, since the residual stress in the compressive direction is applied to the piezoelectric layer 201 in advance, the tensile stress applied due to the vibration is reduced. However, since the piezoelectric layer 201 is initially warped and the residual stress in the compressive direction is extremely small in the area near the surface of the piezoelectric body, there is a risk that a crack will be formed in the surface the piezoelectric body and the piezoelectric body will break.
In addition, in a piezoelectric pump in which a wall surface of a pump chamber is formed by a vibrating plate and the vibrating plate is vibrated by the piezoelectric element or the piezoelectric thin film to convey fluid in the pump chamber, there is a risk that the fluid cannot be conveyed if the piezoelectric layer breaks.